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Part V: Techniques and Theory
A. Extraction (See also, Padías pp. 116-127)
Sample Extraction Procedure for Separation of a Mixture of an Acid, a Base and a Neutral
Compound by Chemically Active Extraction
This procedure is designed to separate a ternary mixture of organic compounds using acid and base
extractions. This example is provided to give you some guidance for both acidic and basic extraction
procedures.
Procedure:
Be sure to record all pH measurements, all weights, mp's and Rf values of isolated products in
your Results Section. Be sure compounds are dry before weighing or taking m.p.'s.
1. Dissolution of Sample.
Weigh out, to the nearest thousandth of a gram, approximately 2 g of the mixture. Dissolve
the mixture in ~50 mL of dichloromethane (CH2Cl2). If any solid doesn't dissolve, add an
additional 25 mL of CH2Cl2. If undissolved solid still remains, filter (by gravity) the solution
directly into a separatory funnel. Otherwise, pour the solution into the separatory funnel.
2. Acid Extraction.
Extract the CH2Cl2 solution two times with 25 mL aliquots of 1.0 M HCl. Combine the
aqueous layers from both extractions in a labeled 250 ml beaker and test the pH of the
aqueous solution. It should be as acidic as the original 1.0 M HCl solution. If it is not,
extract the CH2Cl2 solution again with 25 mL of 1.0 M HCl.
3. Basic Extraction.
Extract the CH2Cl2 solution resulting from step 2, two times with 25 mL aliquots of 1.0 M
NaOH. Combine the aqueous layers in a labeled 250 mL beaker and test the pH of the
aqueous solution. It should be as basic as the original 1.0 M NaOH solution. If it is not,
extract the CH2Cl2 solution again with 25 mL of 1.0 M NaOH.
4. Recovery of the Compounds.
CH2Cl2 solution:
Dry the CH2Cl2 solution resulting from step (3.) with anhydrous Na2SO4. Let this
mixture stand for about 5 min., swirling it frequently. Remove the drying agent by
gravity filtering the solution into a 125 ml Erlenmeyer flask, add a boiling stone and
evaporate the solvent in a hot water bath in a hood. Transfer the residue to a watch glass
and allow it to dry.
Acid Extracts:
While the CH2Cl2 is evaporating, cool the acid solution from step (2.) in an ice bath and
then make the solution basic by slowly adding 6 N NaOH until the solution tests strongly
basic with pH paper. A precipitate should form during the addition of NaOH. Collect the
solid by vacuum filtration and wash the solid on the filter with cold water. Transfer the
compound to a large filter paper and allow it to dry.
Base Extracts:
Cool the basic solution from step (3.) in an ice bath and then make the solution acidic by
slowly adding 6 M HCl and stirring until it tests strongly acidic with pH paper. Collect
the precipitated solid by vacuum filtration and wash the solid on the filter with cold water.
Transfer the compound to a large filter paper and allow it to dry (see section [d.], below).
V A. Stereoisomers
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Drying Procedures:
The compound recovered from the CH2Cl2 solution should be crushed to small particle
size and allowed to dry for a total of about 1 hour. It will dry quickly since it was isolated
from a low-boiling solvent.
The compounds recovered from the acid and base extracts should be allowed to air dry in
your desk until the next period. These compounds were isolated from water and water
evaporates slowly from these polar compounds.
5. Sample Characterization and Submission:
Determine the mass of the compound recovered from the CH2Cl2 solution. Verify the
identity of the recovered compound by TLC analysis vs. authentic samples of the original
components using an appropriate developing solvent system. Finally, determine the mp of
each recovered compound side-by-side with an authentic sample of the compound. Record
your data in your Results Section, place your sample in an appropriately labeled vial (See Lab
Manual Part II. p. 15), and seal it tightly. Submit your sample according to your instructor’s
directions.
Next Lab Period:
After the compounds you recovered from the aqueous solutions have had sufficient time to
dry completely, determine their masses and verify their identity by mp and TLC analysis vs.
authentic samples, label, and submit the samples as with that recovered from CH2Cl2.
Sample Flow Diagram
Flow Diagram for Separation of the Mixture of an Acid, a Base and a Neutral Compound
described in the procedure on B-1. (See also, Lehman pp. 289-290)
Abbreviations
A-H
B:
=
=
acid
base
N
DCM
=
=
neutral compound
dichloromethane
Original Organic Phase
A-H, B:, N, DCM
Extract w/ 1M HCl
aqueous
+
B-H
+
H3 O , Cl
-cool
-basic w/ 6M NaOH
aqueous
precipitate
B:
H2 O , Na
Cl ,
-vacuum filter
-wash w/ cold water
-air dry
+
OH
aqueous
-
A
, Na
+
-
H2 O , OH
-cool
-acidic w/ 6N HCl
precipitate
A-H
B:
organic
A-H, N
DCM
H2 O (trace)
Extract w/ 1M NaOH
organic
N
DCM
H2 O (trace)
-vacuum filter
-wash w/ cold water
-air dry
A-H
-dry w/
anhy. Na2SO4
aqueous
+
H2O , H3O
organic
solid
+
N
Cl , Na
Na 2SO4.H2 O
DCM
evaporate solvent
N